Alloys: composition, diagrams and useful properties
Recognise mixed atom types and connect brass and stainless steel with their stated composition and uses.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Distinguish a mixture from a compound.
- Separate hardness, strength and resistance to rusting.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is the required brass composition?
What you will learn
- Define alloys as mixtures of a metal with other elements and recall brass and stainless-steel examples.
- Identify alloy diagrams and connect potential hardness, strength and corrosion resistance with uses.
The mixture can include a non-metal
An alloy is a mixture of a metal with other elements. Brass contains copper and zinc. Stainless steel contains iron and other elements such as chromium, nickel and carbon; composition varies between grades, so those names do not state one fixed molecular formula for every stainless steel. Carbon demonstrates that an alloy need not consist only of two metals.
In the schematic structural comparison, the pure metal contains one represented atom type, while the alloy contains another type mixed into the metal. The different symbols or sizes identify a mixture rather than a pure-metal diagram. A row of unlike symbols is not automatically one bonded molecule: read the diagram's key and context. The Extended lesson explains how size differences affect deformation.
Useful does not mean harder in every possible mixture
Alloys can be harder and stronger than the corresponding pure metals and can better serve a particular use. Stainless steel is used in cutlery because of hardness and resistance to rusting. Hardness helps resist scratching or indentation, while corrosion resistance helps preserve the material during suitable use. These are distinct reasons rather than interchangeable words for high density.
An alloy's properties depend on composition and treatment, so can be harder does not mean every possible alloy outperforms every pure metal in every property. Brass and stainless steel are examples of selecting a mixture for useful properties, not fixed-ratio ionic compounds. Describe the requirement and the relevant alloy behaviour rather than assuming mixing any substances produces the same improvement.
Pause and explain
Which diagram evidence supports the stated alloy identification?
Worked example
A diagram contains mostly identical metal atoms with some differently represented atoms mixed among them. Explain the alloy identification and give the required brass composition.
Show the worked solution
- The mixture of represented atom types distinguishes the schematic alloy from a pure metal.
- Brass is a mixture containing copper and zinc.
- The alloy category does not imply a fixed molecular formula or that only metallic additions are possible.
Answer Mixed atom types support an alloy diagram; brass contains copper and zinc.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define alloy and give the required brass and stainless-steel composition examples.
Give me a hint
Keep the mixture definition wider than two metals only.
Compare my reasoning
- An alloy is a mixture of a metal with other elements.
- Brass contains copper and zinc.
- Stainless steel contains iron with other elements such as chromium, nickel and carbon, depending on grade.
Alloy: metal mixed with other elements. Brass: Cu/Zn. Stainless steel: iron with additions such as Cr, Ni and C.
Look for these in your work
- The definition permits non-metal additions.
- Both examples keep the correct main materials.
Compare the pure-metal and alloy schematic diagrams and explain what evidence distinguishes them without relying only on colour.
Give me a hint
Use the particle sizes/types and the diagram key.
Compare my reasoning
- The pure-metal panel represents one atom type.
- The alloy panel has a second represented type mixed into the metal.
- Different sizes as well as colours provide the identifying key; this establishes composition rather than a universal property value.
One represented atom type versus mixed types distinguishes the schematic pure metal and alloy.
Look for these in your work
- The diagram evidence is explicit.
- Colour alone is not the only distinction.
A report says stainless steel is useful for cutlery solely because it is dense, and says carbon cannot be part of an alloy because it is a non-metal. Correct both claims.
Give me a hint
Use the required physical-property reasons and the full mixture definition.
Compare my reasoning
- The required cutlery reasons are hardness and resistance to rusting.
- Density alone does not explain those two advantages.
- An alloy can mix a metal with other elements, including carbon in an iron-based composition.
Cutlery uses hardness and rust resistance; alloys can include a non-metal such as carbon.
Look for these in your work
- The use is connected to its relevant properties.
- The definition is not restricted to metals only.
Common mistakes
- Defining every alloy as exactly two metals.
- Giving brass the composition iron and carbon.
- Treating every stainless-steel grade as one fixed compound formula.
- Using density as the explanation for both hardness and rust resistance.
What can you explain now?
Which required properties explain stainless-steel cutlery?
Compare with the explanation
Hardness and resistance to rusting.
Hardness supports durable use, while rust resistance preserves the material in its suitable environment.
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Make it stick
Identify an alloy diagram from its key, recall brass and stainless-steel compositions, and give the two cutlery property reasons.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.